Phosphodiesterase (PDE) inhibitors are known to be effective in treating inflammatory diseases. In particular, PDE4 inhibitors are effective in treating inflammatory diseases of respiratory tract including asthma or airway obstruction (e.g., chronic obstructive pulmonary disease (COPD)) (Patent Literature 1). Among them, N-(3,5-dichloropyrid-4-yl)-3-cyclopropylmethoxy-4-difluoromethoxybenzamide (also referred to as roflumilast), which is a PDE4 inhibitor, is commercially available as an oral agent (trade name: Daxas (R)) in Europe and the United States (Patent Literature 2) and is recently investigated for possible application to aqueous medicinal preparations and transdermally absorbable preparations (Patent Literatures 3 to 5).